The present invention relates to a process for rehabilitating sewers that are in danger of collapsing, in particular profiled sewers.
Thousands of kilometers of sewers have existed, unnoticed, for decades beneath heavily travelled traffic surfaces. For some time now, the state of many sections of these underground arteries has made reconstruction and renovation a matter of urgency. In the area of Vienna alone there are some 450 kilometers of profile sewers which, in the opinion of experts, are either leaky or else in danger of collapsing. However, conventional repairs would require an unreasonable amount of additional excavation work, and so it is essential to develop construction methods that will make it possible to perform all the work without any major disruption of life "on the surface."
With regard to the causes of damage and the overall picture of such damage, one has to proceed from the fact that each sewer is subjected to loads that act on it externally as well as loads that act on it internally. External loads are those caused by ground burden, traffic loads, lateral ground pressure resulting from shifting caused by excavations or explosions (aerial bombs) in adjacent areas, settling, and other factors. Internal factors are those such as erosion of the bottom by debris, various types of corrosion caused by aggresive waste water and vapours. The combined effect of all these factors----exacerbated in some cases by deficiencies in planning and execution----leads to the fact that the sewer becomes progessively damaged until it is in need of repair, and has to be renovated. Very frequently today, the spectrum of loads has changed completely relative to the conditions that existed at the time the original work was carried out. Even though ongoing maintenance operations are carried out at many places throughout the sewer network, many areas have become obsolete. If one inspects the sewers to determine their condition or to find visible damage, one is confronted by a very varied and complex picture of the damage that is present: cracks and fissures in the walls, partial or complete erosion of the bottom, loose joints in the masonry, changes in gradients due to subsidence, damaged inflows (disemboguements) and manhole connections, deformation of the profile, and so on. The extraction of core samples provides information about the overall cross-section of the walls and permits one to draw conclusions about the strength of the sewer structure, insofar as this cannot be done from the picture presented by existing damage.
In a sewer that has become damaged in this way, the question of reconstruction or renovation then arises. Whereas, as a rule, reconstruction only provides a temporary solution, renovation or rehabilitation must ensure full functional capacity of the sewer for many decades. This will affect the costs, which will have to be estimated for the construction measures in each case.
The object of all sewer reconstruction must be to reestablish the solidity of the sewer and its resistance to chemical attack. The measures will perforce depend on the shape and size of the sewer cross-section. Much will depend on; whether or not there is room to work within the profile, whether or not equipment will have to be used from shafts, whether or not the existing cross-section is to be retained or enlarged, and whether or not a reduction of the cross-section is acceptable. The causes of existing damage and the picture of existing damage supplement the technical parameters, so that in the final analysis, when economic considerations are also factored in, reaching a decision in favour of a particular reconstruction process can be extremely difficult.
If a slight reduction of the profile is acceptable and there is sufficient working space available, the surrounding ground can be consolidated stability (steadiness with regard to shock and vibration) can be increased by using (cement) injection and the sewer walls can be repaired by using reconstruction products (mainly using a cement-plastic base). The cleaning of the inside walls of the sewer which is required for this is, in most instances, rendered extemely difficult by the presence of a hard layer of sewer scale, which reduces or prevents adhesion of the reconstruction layers that are installed. This also applies in the case of sprayed concrete coatings. Thus, it is essential to reinforce these and build them up to be self-supporting pipes.
A pipe is also formed in the same way in the case of smaller----slippable----profiles, this being done by using an inner shell of steel or in the form of a tube. The sole of this pipe can be of prefabricated plastic-cement skinning. In the case of circular profiles, it is possible to use thin-wall plastic pipe (e.g., PVC or PE), when the remaining intervening space between the new and the old sewer is filled with cement suspension.
When these processes are used, difficulties are caused by the inflow junctions from domestic connections, insofar as these are not fed in through shafts.
Finally, there are procedures in which components of plastic, glass-reinforced plastics, or fibre cement are used to produce a new profile in situ. These require an appropriate amount of working space and, for this reason, are suitable only for larger cross-sections.
The renovation of a sewer can also be done when there is a need to increase its capacity and cross-section. Here, the pipe-advance method is available which makes it possible to advance in two directions from a shaft, so that a shaft is needed for every 200 to 300 meters of sewer. This method is restricted to circular cross-sections and the pipes are advanced through the existing sewer, which is then dismantled section by section. The high level of technical expenditure results in high costs, so that this method must be confined to special cases.
Especially worthy of note are reconstruction methods that employ modern plastics technology. They involve the (mechanical) production of a pipe in the existing profile. The pipes are self-supporting and bonded by artificial resin. As far as is known, this technique is presently only in the testing stage.
In summary, all formerly known and used methods of reconstructing damaged sewers can only be used in certain areas when the damage is not too great. As a rule, the associated costs are extremely high and only offer the possibility of a permanent improvement in a few cases.
Thus, there is still a need for a reconstruction method that is as far as possible universally applicable for repairing damaged sewers, which provides an economical and permanent renovation of damaged waste water lines and which provides the greatest possible continuation of sewer operation, with only slight inconvenience to adjacent properties and the briefest disruption of vehicular traffic.